1. Field of the Invention
This invention relates to liquid nitrogen refrigeration systems, and more particularly to an improved temperature control system for maintaining the desired temperature in a liquid nitrogen refrigerator above the level of the liquid nitrogen refrigerant.
2. Description of the Prior Art
Liquid nitrogen refrigeration systems are well known and widely used for the cryogenic storage of biological specimens and other materials and for industrial or other uses where very low temperatures are required. Such refrigerators conventionally employ a cylindrical stainless steel vacuum insulated inner container defining the specimen storage and the liquid nitrogen refrigerant compartment, with an insulated lid or door providing access to the interior of the refrigerator from the top. Liquid nitrogen is admitted into the bottom of the insulated chamber and specimens to be stored are inserted through the lid.
The level of liquid nitrogen in a typical liquid nitrogen refrigerator is maintained by a liquid level controller that opens a solenoid actuated valve when the liquid level falls below a predetermined low level, and closes the valve when the level reaches the predetermined high level whereby a supply of liquid nitrogen is continuously maintained in the bottom of the chamber. Depending upon the nature of the specimens, storage may be either in the "liquid phase" wherein the specimens are submerged in liquid nitrogen, in which case the liquid nitrogen level is maintained near the top of the chamber, or in the "vapor phase" above the surface of the liquid nitrogen in the inner container, in which case the liquid nitrogen level is maintained at a relatively low level. The temperature of the liquid nitrogen is, as known, constant at -196.degree. C. whereas the temperature of the gaseous nitrogen above the liquid will vary from -196.degree. C. at the liquid-vapor interface to a substantially higher temperature which, for vapor-phase storage, may be within the vicinity of -100.degree. C. to -120.degree. C. directly beneath the closed refrigerator lid.
Ideally, for vapor-phase storage, the temperature within the specimen storage area would be maintained substantially uniform at or below the required storage temperature. In the past the accepted way of reducing the temperature in the top area of the storage space was to raise the level of liquid nitrogen in the inner container. By raising the liquid level, however, specimens stored in the lower level space would be submerged in liquid nitrogen which may not be acceptable.
One method to improve the temperature differential in the vapor-phase storage area has been to install a heat conductive device in the form of an aluminum cylinder or sleeve adjacent the inner surface of the inner container. To be most effective, the sleeve should extend through the specimen storage area. It may or may not extend into the liquid nitrogen depending on the storage temperature required. Such conductive sleeves are only partially effective in reducing the temperature gradient in the vapor-phase storage area, and do not by themselves assure that the desired temperature is maintained. It is, accordingly, an object of the present invention to provide an improved method of and apparatus for providing automatic temperature control in the vapor-stage storage area of a liquid nitrogen refrigerator cryogenic storage system while maintaining the level of the liquid nitrogen within a desired or acceptable range.
Another object is to provide such a method and apparatus for reducing the temperature differential between the upper and lower regions of the vapor-phase storage area by producing turbulence in the gaseous nitrogen above the liquid nitrogen refrigerant in the bottom of the refrigerator.
Another object is to provide such an improved liquid nitrogen refrigerator in which the admission of nitrogen into the refrigerator container is controlled to produce turbulence in the vapor-phase to thereby provide a more uniform temperature throughout the vapor-phase storage area.